1. Field of the Invention
The present invention relates to the lithography technology. More particularly, the present invention relates to a method for checking a phase shift angle of a phase shift mask (PSM), to a lithography process utilizing the method for optimization, and to a phase shift mask of which the phase shift angle can be derived based on the method.
2. Description of the Related Art
In accompany with the requirement in increasing the integration degree of IC devices, the linewidths of advance semiconductor processes are mostly below the wavelength of the exposure light. When the linewidth is reduced to about a half of the wavelength or below, a phase shift mask is required to improve the resolution of the pattern transfer. Generally, a PSM utilizes a phase angle difference between adjacent light-transmittable regions to reduce the light amplitude at the portion of the photoresist layer requiring insufficient exposure to decrease the exposure dose thereat. Thus, the exposure contrast can be improved to enhance the resolution.
However, because the phase shift angle of a PSM is controlled usually by adjusting the substrate or film thickness in the transparent regions but the thickness is difficult to precisely control, the phase shift angle easily deviates from the required one. Consequently, the resolution is lowered and errors are caused in some characteristic values of the lithography performance including the pattern positions and/or the focus center as well as the depth of focus (DOF) and/or the critical dimension (CD) of the photoresist patterns. To solve the problem, a PSM has to be checked for its phase shift angle after being received from the vendor, wherein a proximity-type probed is usually used to measure the substrate or film thicknesses in the regions of different phase angles. If the phase shift angle is found to be erroneous, the exposure conditions can be adjusted accordingly to compensate the error in phase shift angle, or the PSM is returned to the manufacturer, who will calibrate the PSM and fabricate a new one accordingly.
Nevertheless, the above method of using a proximity-type probe to measure the thicknesses not only consumes much time, but also possibly damages the surface of the PSM to degrade the quality of pattern transfer.